Method for deironing chromium ore



Sept. 7, 1948. A. GREFFE 2,448,882

l METHOD FOR DEIRONING CHROMIUM ORE Filed July 4, 1945 Patented Sept. 7, 1948 :METHOD FOR DEIRONING CHROMIUM ORE Andr Greffe, La Bathie, France, assignor to Societe DElectro-Chimie, DElectro-Metallurgie et des Acieries Electriques DUgine, Paris,

France, a corporation of France Application July 4, 1945, Serial No. 603,278 In France December 13, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires December 13, 1963 2 Claims.

Chromium ores usually contain a considerable proportion of iron. The problem of removing the iron from the said ores is set in a large number of cases and more particularly when one intends to obtain the chromium from the ore in a pure or nearly pure condition.

The thermo-chemical methods fwhich are used for carrying out this deironing operation consist essentially in placing in an electric furnace the ore to be treated mixed in a judiciously dosed proportion with a suitable reducing agent and carrying out in said furnace the reduction of the iron contained in the ore, thus removing the iron in metallic form.

Reducing agents which are preferably used for this operation are carbon and silicon. Owing to the equilibrium existing between the iron and the chromium contained in the metallic phase on one hand and the corresponding oxides of the molten ore on the other hand, it is necessary, if one wishes to obtain a very low iron oxide content in the ore, to reduce a portion of the chromium oxide of the ore. This involves a decrease in the chromium oxide content of the thus deironed ore, which renders the following operations, for instance the extraction of the chromium in a pure condition, more difficult and more expensive.

In the case of the use of carbon as a reducing agent the determination of the quantity of carbon which is strictly necessary is diflicult. One is lead to use an excess of reducing agent in the mixture, thus giving rise to a complementary reduction of the chromium oxide and a still more marked impoverishment of the ore in that oxide.

The use of silicon shows a different kind of inconvenience: It gives rise, in the course of the reduction, to the formation of a considerable quantity of silica, which it is necessary to saturate with lime if one will avoid a considerable silicon content in the chromium obtained from thus deironed ore. This silica and the lime which is necessary for saturating it still further dilute the chromium oxide contained in the slag which has formed in consequence of the deironed operation.

The present invention relates to a method by which one can avoid the above mentioned inconveniences. This method consists first in partially reducing the chromium ore, preferably in the presence of a ux, as for instance lime, by means of metallic chromium and in separating from the overlying slag the metallic phase rich in iron formed in the course of this operation and then in allowing the slag to freeze, thereafter crushing it and submitting it to a magnetical sorting operation effected by means which are commonly used for this purpose in the treatment of ores.

In the course of initial reduction the chromium reacts with the iron oxide contained in the ore, thus giving rise, on one hand, to a ferro-chromium which is comparatively poor in chromium and, on the other hand, to chromium oxide which passes into the slag. This slag itself contains then Ionly a very low proportion of iron oxide.

It has been found, however, that after reduction by chromium of the iron oxide contained in the ore a comparatively large proportion of ferro-chromium formed in the course of the reduction remains in the final slag, said ferro-chromium being dispersed in the slag in the form of fine drops. As the ferro-chromium which has been obtained is rich in iron, these fine drops form a considerable part, which can reach up to 90%, of the total iron content of the slag, the remaining small proportion of the iron content of the slag being in the form of iron oxide. Now it has been found that the ferro-chromium obtained in the course of reduction has a composition lwhich is such that the said alloy shows magnetic properties which precisely make it possible, according to the invention, to separate from the slag, after crushing it, the ferro-chromium contained therein'.

In practice, the chromium used as a reducing agent in the method according to the invention, need not be in the condition of a pure metal; it is also possible to work with a ferrochromium alloy having a comparatively high chromium content, for instance Of course, the chromium content of the ferro-chromium alloy obtained after the reduction operation is lower than that of the initial alloy, but the said ferro-chromium alloy has a chromium content such that it can be commercially used as low ferro-chromium. The quantity of ferro-chromium used as reducing agent must be calculated as a function of its chromium content in order to obtain a ferro-chromium alloy containing more than 20% of chromium at the end of the reduction operation.

On the other hand, the present invention provides for the use of the hereabove dened principal method for the obtention of ferro-chromium having a very high chromium content or even of industrial metallic chromium, for instance thru a largely extended reduction of deironed slag by means of a reducing agent. This reduction can be effected, for instance, in a 3 known manner, by aluminothermy or by means of carbon.

It is also possible to reduce the deironed slag in two stages by use of a similar method to that which is described in my U. S. patent application No. 603,279, filed July 4, 1945.

To this intent, in the course of the first stage, a silica-chromium with a low silicon contentl is first caused to act on deironed slag with or without a violent intermixing of the reacting elements and in the presence of a base such as lime, thus giving rise, on one hand, to formation of very rich ferro-chromium or to metallic industrial chromium and, on the other hand to formation of a slag containing silica, lime and chromium oxide, said slag being submitted, in the course of the second stage, to the action of a Silico-chromium with a high silicon content, prepared by the action of silica and carbon on a portion of the deironed slag to be reduced, this second stage, which is preferably effected by a violent intermixing of the phases present, giving rise, on one hand, to a slag which contains only silica and lime and which is removed and, on the other hand, to a Silico-chromium which can be used in the'flrst stage of reducing a further quantity of deironed slag.

The invention is illustrated by the accompanying flow sheet. rThe following is an example of one way in which the method according to my invention may be carried out:

Example-A mixture of 300 kg. of chromium ore containing 48.90% of Cr2O3 and 14.34% of FeO with 300 kg. of lime is melted in an electric furnace I. When the melting is complete, 156 kg. of ferro-chromium containing 67.50% of chromium is charged onto thev surface of 'the bath. After the reaction has proceeded for a sufficient time, the metal 2 and slag 3 are tapped separately from' the furnace, the slag amounting to 587 kg. andv the metal amounting to 94 kg. The slag and. metal. had the following compositions:

Slag: Percent Total Fe (metallic Fe-l-Fe-oxide) 3.00 CrzOs 40.80 S102 2.00 A1203 12.00 CaO 31.80 MgO 10.23

Metal:

Cr 31.58 Fe 67.80

The slag was allowed to solidify, was crushed and was subjected to a magnetic sorting operation in a magnetic separator 4. The thus deironed slag contained:

Percent Total Fe (metallic Fe-l-Fe-oxide) 0.28

CrzOs 39.88

4 slag. The ferro-chromium 6 'was returned to the furnace l for reducing another batch of chromium ore. The slag 1 from the mixer 5 was transferred to a mixer 8 and Silico-chromium 9 having a high ratio of silicon to chromium was added to the mixer 8. The Silico-chromium 9 was made by mixing in a furnace l0 a part of the deironed slag coming from the magnetic separator 4, together with carbon and silica. After violent intermixing in the mixer 8, the slag II was discarded and the silice-chromium I2 having a low ratio of silicon to chromium was returned to the mixer 5 and used for treating another' batch of deironed slag from the magnetic separator 4.

What I claim is:

1`.. A process for the production of a chromium alloy containing less than 2% iron from natural iron and chromium ore, which comprises melting said ore and reducing it by metallic chromium in the presence of lime, thereby producing on the one hand an overlying layer of molten slag having a chromium oxide content higher than that in said ore and an iron content lower than that in' said ore and on the other hand an underlying `layer of molten ferro-chromium, separating the layer of molten ferro-chromium from the layer of molten slag, allowing the slag to freeze, thereafter crushing it and submitting it toa magnetic sorting operation to remove ferrochromium dispersed in the slag, and submitting this deironed slag to. a reduction operation.

2. A process for the production of a chromium alloy containing less than 2% iron from natural iron and chromium ore, which comprises melting said or=e and reducing it by metallic chromium in the presence of lime, thereby producing on the one hand an overlying layer of molten slag having a chromium oxide content higher than that in said -ore and an iron content lower than that in said ore and on the other hand an underlying layer of molten ferro-chromium, separating the layer of molten ferro-chromium from the layer of molten slag, allowing the slag to freeze, thereafter crushing it and submitting it to a magnetic sorting operation and submitting this deironed slag to a reduction in two stages by rst causing a Silico-chromium with a low silicon content to act on said slag and then submitting the slag obtained in the first stage to the action of a Silico-chromium having a high silicon content, the Silico-chromium with a 10W silicon content obtained in the course of this second stage being used for treating the deironed slag in the course of the rst stage of another reduction operation.

ANDR GREFFE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,136,669 Goldschmidt Apr. 20, 1915 1,520,240 Hamilton Dec. 23, 1924 1,717,160 Kichline June 11, 1929 2,098,176 Udy Nov. 2, 1937 2,127,074 Udy Aug. 16, 1938 2,169,741 Perrin Aug. 15, 1939 2,264,204 Heckett Nov. 425, 1941 

